A portion of a conventional gas turbine engine fuel control system is illustrated in FIG. 5 and comprises a main fuel passage 200 in which a metering valve 202 is disposed for providing a metered supply of fuel to an engine (not shown). The position of the metering valve 202 is controlled by controlling pressures exerted against a first portion 204 of the metering valve 202 to move the metering valve 202 to a desired position; a linear variable differential transducer (LVDT) 206 may also be associated with the metering valve 202 to provide information to a controller (not shown) regarding the position of the metering valve 202. Control pressure may beneficially be provided from either a primary pressure source 208, 210 or a secondary or backup pressure source 212, 214 to provide redundancy if a problem arises in connection with the primary pressure source 208, 210. A transfer valve 216 controls which pressure source is used to control the position of the metering valve 202. A first EHSV 218 is commonly used to control the position of the transfer valve 216.
A pressure drop across the metering valve 202 is regulated by a throttling valve 220, and a shutoff valve 222 is provided downstream of the metering valve 202 to provide a drip tight shutoff of the fuel flow through the fuel control system. The position of shutoff valve 222 is controlled by a second EHSV 224 which selectively ports a pressure via a passageway 226 against one side of the shutoff valve 222 to move shutoff valve 222 to a closed position. Seals 228 provide for drip-tight shutoff stopping all or substantially all fuel flow through the fuel flow passage downstream of the shutoff valve 202.
Reducing the weight of gas turbine engines and their associated control systems is sometimes a factor in gas turbine engine design, especially when the engine is to be used on an aircraft. It would therefore be desirable to provide a fuel control system for a gas turbine engine that has functionalities and a reliability similar to those of known fuel control systems while reducing the weight of the system.